Why do some genera have so many species?

Why are there genera that have thousands of species? Does this not warrant the division of the genus into multiple new ones, or the elevation of the genus to a tribe? I know it’s a lot of work, and that we have many subgeneric ranks such as subgenus and section, but ultimately we have hundreds or thousands of species that are all Xx whatever.
I admit I lean towards lumping. But I only see disadvantages to the current approach. It doesn’t make sense to make hard rules such as “there can only be up to # species in a genus”. What other options are there?

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I agree that it’s unwieldy to have huge genera. I guess it comes from the differing taxonomic “tastes” of the experts who worked on certain groups in the past. I also suspect that for certain groups, you have genera that are easily grouped together by some consistent apomorphic trait, but they don’t readily subdivide into an obvious scheme, so it’s more useful to just keep them as the same genus.

If we were doing it again from the beginning, it would probably be good to lay down some standards or expectations on how large a genus can be before it starts to lose it’s usefulness, but in the present day there are serious downsides to revising genera that don’t occur for other taxonomic groupings. The genus is part of an organism’s scientific name, so splitting up a large genus into multiple more convenient genera creates a kind of taxonomic chaos. For a period of time, until the proposed change is widely accepted (or rejected), researcher and lay people will be referring to species by multiple names, which is confusing. And following adoption of a genus-level change, there is a discontinuity in the literature – pre-revision articles and books will refer to the species by a different, outdated name. Subdividing a genus into subgenera or altering the contents of families doesn’t have this ripple effect, as those names only come up in certain contexts rather than every time scientists refer to the species.

Obviously, if a genus is problematic, it should be revised. But if it’s phylogenetically valid and functional within the field, there’s a lot to be said for keeping genus names the same as they always have been, even if it feels a bit like poor taste.

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Bit of a side yet related question: As someone who has zero academic training in taxonomy and/or phylogeny, I often wonder what is the formal definition of a species? Is there like a threshold percentage difference between the DNA that separates specimens into different species? And then as one goes to higher classifications, does that threshold DNA just increase? This way of thinking though seems too straightforward considering many taxa are being revised and reclassified.

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Your question prompted me to look at what had been written on the topic, specifically for plants. Well, there are all sorts of reasons for lumping genera and all sorts of reasons for splitting them. A good review article is

History and Concepts of Big Plant Genera
Author(s): David G. Frodin
Source: Taxon, Vol. 53, No. 3 (Aug., 2004), pp. 753-776
Available via this link: https://iks.ukzn.ac.za/sites/default/files/4135449.pdf

From a quick skim of that article I learned that the issues of “What is a genus?” and “How big is too big?” have been around since the days of Linnaeus. Linnaeus detailed 56 species of Euphorbia in the first edition of Species Plantarum, but also believed that genus should have no more than 100 species. Expansion after that point tended to proceed based on where collection was happening – mostly with genera from temperate regions at first, then with those from colonial possessions.

In general, botanists stuck to the concept that all species with a consistent “character” formed a genus. By the 1880s there were 28 plant genera that contained more than 300 species. There were various attempts to split the larger genera, but the principal obstacle was a lack of clear differences that could be used consistently. The paper looks at Rhododendron as an example of a large genus that could not easily be split and Cassia as an example where floral and seedpod form allowed separate genera of Chamaecrista, Senna and Cassia to be described.

In the 20th century, there were renewed concerns about the unwieldy nature of big genera preventing comprehensive analysis or easy keying. But (to my mind) a lot of the lumping vs. splitting arguments of this period were ultimately futile because the botanists involved had no access to data on the underlying genetic relationships. That started to changed with techniques such as karyology and since the 1980s has undergone significant shifts through phylogenetics.

Ultimately, though, it’s not just enough to say “This genus is too big”. It’s also necessary to be able to identify two or more separate clades that can be conclusively separated using a consistent combination of characters.

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Haha robotpie, that is an extremely difficult question! There are multiple different ways to define species, none of which are applicable in all circumstances, and some of them disagree in the same circumstance. Conceptually, a species is a group of organisms that are sharing genes with one another, but not with other organisms, and therefore are evolving together on an independent trajectory. How do you measure that??

Traditionally, species were delimited by appearance (“morphology”) and by their unwillingness to mate with other similar organisms (this latter method is confusingly called the “biological” species concept). But those methods kind of fall apart for certain kinds of organisms. Most unicellular organisms have very little in the way of morphology, and it has turned out that sometimes very similar looking organisms belong to separate “cryptic” species. On the other hand, you can’t really use mating preferences to split up asexual organisms or ring species. The ability to sequence DNA has revolutionized taxonomy, but, as you say, you can’t usually just measure some percentage of genetic difference and stamp them “approved species.” (and you can’t sequence most extinct organisms either!)

One thing modern studies do is try to use genetic sequencing to infer the flow of genes across the landscape. Where different groups are genetically distinct, and the barrier between them fairly discrete, we call them different species. Where there is a smoother gradation in genetic make-up, we may lump lots of organisms in the same species. Here’s a twitter thread about a study that did just that in American and Northwestern crows!

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There are cases where splitting genera requires a lot of new genera to be created to significantly reduce the number of species in the genus.

Because all genera must be monophyletic basal groups must get their own genera if the crown group is to get its own genus. Take a look at Asteraceae subfamilies for example, they’re not genera, but each subfamily must be monophyletic, so the same principle applies. To split off the massive 16,000+ species Asteroideae from the rest of the family 11 basal radiations have to be elevated to the level of subfamily, and Asteroideae still contains 70% of Asteraceae diversity. Many of these subfamilies are monotypic or only have a few species, but to keep the groupings monotypic they must be elevated as well.

This isn’t the case for all genera but how many new genera in a family is worth it to break up a big genus. If Euphorbia could be reduced to 1500 species how many new genera would make it worth it to do so? If there are extensive basal radiations breaking up a genus may require many genera, and many monotypic genera to significantly reduce the species count. (I don’t know if this is the specific case for Euphorbia its just my go to big genus)

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Thanks for asking, I’ve wondered about this as well. I’m used to birds where all the species in one genus have basically the same shape and have shared colouration patterns etc., but then I started learning about plants and Drosera has 200 species ranging from the classical tiny rosettes to crazy vine-like species, and what even is Euphorbia!? I guess I get the appeal of it being tradition and what everyone remembers but those don’t seem like priorities to me. I figured taxonomists focusing on different taxa just have different definitions.

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Cyathia. And that’s about it lol

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Well, Euphorbia was brought up so I guess now I have to comment! :-)

I’ve been in favor of breaking up Euphorbia for a long time and Chamaesyce, Pedilanthus, Monadenium, Synadenium, Tithymalus, and others have been recognized. This taxonomic battle has been going on since at least the early 1800s and became quite a back and forth during the 1900s with various experts either deciding to lump all but the more divergent forms together or split out dubious groups. Few considered all genera that are now in Euphorbia to be in one genus but preferred subfamilial taxa like tribe or subtribe (see Euphorbiinae). There are several things we now know that get in the way, though.

The most basic way to explain the situation is that you have a body plan shared throughout much of the genus with many alterations from and reversions back to that form (viz. subg. Esula and sect. Nummulariopsis) in all four major clades such that there is little cohesion except at monophyletic tips where noval characteristics have developed (like C4 photosynthesis Chamaesyce/sect. Anisophyllum or spurred cyathia in Pedilanthus/sect. Crepidaria). In other words, you can recognize the noval groups at genus level, but only if you recognize two groups that have very similar body plans as different genera too. In otherwords, if the more conserved members across three of the four clades went extinct, you might be able to split them easy enough. But with them extant, you can’t really do that cleanly.

A more familiar example is the problem of recognizing birds at the same taxonomic rank as reptiles when birds “are” reptiles in that they descended from and are nested within reptiles. The phylogenetic term is paraphyletic. Personally, I’m one of a those weird taxonomists that think it might be worth accepting paraphyletic taxa, but the vast majority frown on that position.

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I was thinking of Euphorbia when I asked the question so I’m kind of pleased others brought it up… hehe…

and while I love birds, as a plant person, it feels (relatively) as though there’s like three of them

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I am still mad this went back to Euphorbia

sounds good to me

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Some species have minor differences and evolved pretty recently yet there can be hundreds of those in the genus, if they’re closely related there’s no need to split the genus. With time of some million years it would make more sense to do so.)
Regarding birds they’re reptiles, there’s nothing that separates them, they’re the same as other dinos and have no truly unique traits, it’s just how human sees them through science history.

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I laughed so hard at this!

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The most accurate reply is, “well, it’s complicated.”

There are a bunch of potential ways to define a species, but each fails in some manner or another. As a result there are a lot of potential ways to define species.

An example of the way a definition of species may fails is to look at the most commonly taught one, the Biological Species Concept, which basically states that if individuals can produce fertile offspring together then both belong to the same species. Well, this definition fails when it comes to asexually reproducing organisms, and there are too many exceptions to it. For example llamas and camels are clearly different (although related) species, no-one would claim them to be the same species, but they can produce fertile offspring (assisted fertilization, male camel, female llama). Macaques frequently hybridize producing fertile offspring, both anthropogenically and naturally at the margins of their species ranges, with rhesus/pig-tailed macaque and rhesus/long-tailed macaque hybryds being especially common. This sort of thing has increasingly been found to be the case in animals and is far more common in plants. As a result of these exceptions and problems, despite the prevalence of this definition being taught in school and in undergrad it’s not really used all that much in professional circles any more.

Here are a couple of incomplete lists of some of the ways species are presently defined:

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As an evolutionary biologist, I have never encountered any definition of a genus other than “a group of species closely related enough to be considered by taxonomists to be of the same genus.” In other words, there is no modern scientific concept behind ‘genus.’ There are thousands of definitions of species, and even when these don’t contradict each other (they usually do), they are often difficult to apply and arbitrary. But as far as I know a genus (as well as a tribe, a family, etc.) is entirely up to the preferences of the community of specialists in that area.

The variety of biological processes, biological outcomes, is far too great for any of our definitions to consistently fit soundly or make sense.

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@chrisangell and @earthknight I appreciate your responses.

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Oho, a thread about massive genera that need to be split?

Bulbophyllum, Epidendrum, Maxillaria, Pleurothallis, and Oncidium have entered the chat

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Oh man, the talk about Euphorbia reminded me of the controversy around the genus Acacia, specifically the classification of Acacia species in Africa versus the Australian wattles. I think, personally, that this was one of the worst cases of vote rigging and biased academic red tape I have seen, but then I am a layman and there isn’t much I can do about it.

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there are some phenomena acting on the number of species/taxa for a certain genus:

  1. Evolutive history (what now has few taxa/species could have had more in the past and vice versa)
  2. Environmental/geological history (isolation and environmental conditions are powerful speciation drivers)
  3. Reproduction strategies (think about genera in which agamospecies are frequent)
  4. Attitude of scientists towards splitting or lumping (some genera that have undergone a strong splitting would need a taxonomic reconsideration)
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In any speciose genus, you’re likely to find some subsets of the group that clump together more closely than others in terms of characteristics or genetics. So the subgenus is often used to bring some degree of order to the oversized genus. But taxonomists seem not to like the subgenus category all that much. Most subgenera are either ignored or eventually elevated to full genus. This has been true in chipmunks in which there was recognized anywhere from one genus (Tamias) with three subgenera to three genera (Tamias, Eutamias, Neotamias) with no subgenera. The sciurid taxonomists seem to have settled on the latter arrangement. But there really isn’t much functional difference between a subgenus and a genus except that you can ignore the subgenus if you wish when using the scientific name.

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